The present invention relates to the field of data communications networks, in particular to broadband access systems.
The World-Wide-Web and the increasing demand for computer power and memory usage of software applications and data files puts tremendous pressure on networking infrastructure. While this pressure can be relieved at moderate cost in the premise Local Area Network (LAN) by migration to Fast Ethernet, LAN Switching or even Gigabit Ethernet technology, the residential local access loop presents a bottleneck. Many technologies such as ISDN, xDSL, HFC, FTTC, and FTTH have been proposed and tested in field trials. Unfortunately, most of these high-speed access technologies are too expensive for the typical residential consumer, and require expensive optical and electronic component technologies. Because of the telephone service providers"" insistence on carrying voice and data over the same wiring infrastructure, priority-based network protocols such as ATM are typically utilized, further increasing the cost of the system.
A potential approach, as identified and addressed by the present invention, is to attempt using Ethernet over a power-splitting fiber-to-the-home (FTTH) network. A block diagram of a generic power-splitting FTTH network is shown in FIG. 1 in which multiple optical network units (ONUs) 10 are coupled to a power splitter 15 which is coupled via fiber 18 to a central office (CO) 20. Attempting to use Ethernet technology in such a configuration, however, is problematic.
In point-to-point links, Ethernet is typically limited to distances of less than 2-3 km due to either modal dispersion (over multi-mode fiber only) or power budget (over single-mode). The power budget limitation arises because no more than xe2x88x924 to +2 dBm of optical power (depending on wavelength) may be emitted by an Ethernet LAN transceiver for eye safety reasons in an intra-building network. Such eye safety limitations, however, do not apply to access networks.
Although the constraints imposed by power budget and modal dispersion considerations can be overcome, the media access scheme of Ethernet imposes significant range limitations which impede scalability to access network applications. When sending data to the CO 20, the ONUs 10 negotiate for media access using the Ethernet carrier-sense-multiple-access/collision detection (CSMA/CD) media access control (MAC) layer protocol. In order for this protocol to operate properly, however, the round-trip delay of each packet may not exceed the duration of the shortest packet. For conventional 10 Mbps Ethernet, this constraint typically imposes a range limitation of a few kilometers. Such a range is inadequate for an access network. As such, the use of Ethernet technology in access networks is not practicable.
The present invention provides an optical data communications network in which the Ethernet media access control (MAC) layer protocol can be used over distances that are much larger than conventionally supported by the Ethernet MAC layer protocol. In accordance with the present invention, upstream and downstream traffic is logically or physically separated on a power-splitting FTTH network. Such an arrangement makes possible the use of ubiquitous and inexpensive Ethernet LAN technology in highly cost sensitive residential broadband access applications.
By logically and/or physically separating the upstream and downstream traffic in accordance with the present invention, the long fiber run 18 between the CO 20 and the power splitter 15 is removed from the CSMA/CD collision domain. The separation of traffic into upstream and downstream directions is consistent with the flow of traffic in access networks which is predominantly between a central office and multiple stations, rather than from station to station.
By removing the long fiber run 18 between the CO 20 and the power splitter 15 from the collision domain, the collision domain is thus made relatively small, thereby allowing greater upstream data rates than would otherwise be possible with conventional Ethernet. Upstream data rates can be increased even further by placing the splitter 15 even closer to the ONUs 10 thereby allowing an even smaller collision domain.
Furthermore, by separating the upstream and downstream traffic and thus removing the downstream traffic from the CSMA/CD collision domain, the downstream data rate is not limited by the size of the collision domain and can thus be substantially greater than the upstream rate. As such, whereas 10 Mbps rates can be provided in the upstream direction, 100 Mbps or greater rates can be provided in the downstream direction. This comports nicely with the typical requirements of access networks.
In an exemplary embodiment of the present invention, an overlay passive FTTH network for data communications to residential customers is provided which can be used, for example, for internet access, working-from-home, pay-per-view, Web-TV, etc. Voice encapsulated in Ethernet packets may also be carried over such a network. Other packet formats such as IP may also be used on the network layer.
While embodiments of the present invention described herein may be referred to as FTTH networks, naturally, the present invention is in no way limited to residential applications and can also be used, for example, in a wide variety of commercial, industrial and institutional applications, among others.